1. Field of the Invention
The present invention relates to flat display screens. It more specifically relates to the transfer of electric connections between the inside and the outside of a tight chamber defined by two plates respectively forming the bottom and the surface of the screen.
2. Discussion of the Related Art
Conventionally, a flat screen is formed of two generally rectangular external plates, for example made of glass. One plate forms the screen surface, while the other forms the screen bottom, for example, provided with the emission means. The two plates are assembled by means of a seal, to be spaced apart from each other. For a field effect display (FED) or microtip screen, or for a vacuum fluorescent display (VFD), vacuum is made in the space separating the two glass plates, while, for a plasma screen, this space is filled with low pressure gas.
To simplify the present description, only microtip screens will be considered hereafter, but it should be noted that the present invention generally relates to the various above-mentioned display types and the like.
FIGS. 1A and 1B partially and schematically show the conventional structure of a microtip screen. FIG. 1A is a cross-sectional view. FIG. 1B is a plane view along line B-Bxe2x80x2 of FIG. 1A.
Such a microtip screen is essentially formed, on a first substrate 1 (FIG. 1A), for example made of glass, of a cathode 2 with microtips 3 and of a grid 4. The cathode-grid is placed opposite a cathodoluminescent anode 5 made on a second substrate 6, for example, in glass, which generally forms the transparent screen surface.
The operating principle and the detail of the structure of such a screen are described, for example, in U.S. Pat. No. 4,940,917 assigned to the Commissariat à l"" Energie Atomique.
Cathode-grid 2-4 and anode 5 are made separately on the two substrates 1 and 6 and are subsequently assembled by means of a peripheral seal or sealing wall 7. Substrates 1 and 6 define an empty space 8 for the circulation of the electrons from cathode 2 to anode 5.
Cathode 2 is organized in columns 9 and comprises, on substrate 1, cathode conductors arranged in meshes from a conductive layer. Microtips 3 are generally made on a resistive layer deposited on the cathode conductors and are arranged inside the meshes defined by the cathode conductors. In FIG. 1A, the cathode conductors and the resistive layer have been designated by common reference 9 identifying different columns. Grid 4 is organized in conductive lines 10 (FIG. 1B) deposited on an insulating layer 11 (for example, made of silicon oxide (SiO2)). Holes 12 are formed in lines 10 of grid 4 and in insulating layer 11 to receive each microtip 3. The intersection of a line of grid 4 and of a column of cathode 2 generally defines a screen pixel.
On the anode side, phosphor elements (not shown) are deposited on electrodes formed in a generally transparent conductive layer and are excited by electrons emitted by the cathode. For clarity, the anode and all its components have been designated by general reference 5 in FIG. 1A.
To properly bias (address) the different conductors of cathode 2, of grid 4, and of anode 5, these conductors have to be accessible from the outside of the chamber closed by a sealing wall 7, generally a fusible glass cord.
A problem which is raised at the passing of the sealing wall is due to the very different characteristics of the materials above and under the wall. This problem is essentially raised for lines 10 of grid 4 which are above the stack on the cathode-grid side.
There is the silicon oxide constitutive of layer 11 with a thickness of about one micrometer, the metal, generally niobium, constitutive of grid layer 4 with a thickness under one micrometer, and the material (generally, fusible glass) constitutive of sealing wall 7 with a thickness of several hundred micrometers. The differences between the intrinsic characteristics of the materials, associated with the high thickness differences between these layers of materials, result in stress which causes cracks in the silicon oxide constitutive of layer 11 of insulation between the grid lines and the cathode columns. Such cracks can extend on either side of the sealing wall and create leaks between the inside under vacuum and the outside of the screen.
The stress essentially results from the different thermal expansion coefficients between the materials forming the stack under the sealing wall. Accordingly, upon addition of the high temperature fusible glass cord, the above-mentioned defects appear.
Another problem arises for lines 10 of grid 4. Indeed, in a conventional screen, the conductive layer constitutive of lines 10 of grid 4 is the last layer deposited on cathode-grid plate 1. Accordingly, the fusible glass constitutive of wall 7 is directly deposited on end sections or extensions 10xe2x80x2 of connection of lines 10 to the outside. Generally, the grid lines are formed of niobium and an etching of the niobium by the glass constitutive of seal 7 is observed. This results in risks of screen malfunction due to the interruption of some lines 10 between the inside and the outside of the screen.
The present invention aims at overcoming the disadvantages of conventional screens in relation with the passing of a conductive path between the inside and the outside of the screen.
The present invention aims, in particular, at providing a novel microtip screen in which any risk of a leak between the inside and the outside of the screen, linked to the passing of the grid conductors under the sealing wall, is suppressed.
The present invention further aims at providing a solution which minimizes the modifications to be brought to conventional methods of microtip screen manufacturing and sealing.
To achieve these objects, the present invention provides an electric connection between the internal space of a flat screen and the outside by means of a conductive line of small thickness underlying a substantially thicker peripheral seal, and including an open-worked structure making its expansion easier, the open-worked structure being such that the electric connection has no rectilinear conductive section.
The present invention further provides a flat microtip screen grid formed of lines deposited on an insulating layer coating cathode conductors, each grid line being provided with at least one electric connection.
According to an embodiment of the present invention, the grid is made of chromium.
According to an embodiment of the present invention, each line is made of niobium in the active screen area, each electric connection being made of chromium.
The present invention further provides a flat display screen of the type including a cathode with microtips; for bombarding a cathodoluminescent anode, and including a grid.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings.